1. Field of the Invention
The present invention relates to an anti-aging additive composition for a quench oil circuit in an ethylene production plant, specifically to control the viscosity of the quench oil (FOK) obtained as bottom product of the fractionating column, and utilized for steam production in the gasifying cracking process to cool down the output from the cracking, and for the commercial production of fuel oil (FOK).
2. Description of the Prior Art
As it is well known, ethylene production for petrochemical use is an important process in the field of oil refinery.
This production takes place in different steps, starting from an ethane, virgin naphtha or gas oil feed, that is subject to gasifying cracking, by means of high temperature steam injection, in a reactor where temperatures reach values in the range of 750-900.degree. C.
This output is cooled by mixing it with a colder fluid and fed in a fractionating column where the separation of ethylene and light gasolines from a heavier oil (FOK) takes place.
The quality of the distillation, i.e. the amount of ethylene, light olefins and gasolines extracted from the top of the column, is influenced by the temperature of the feed in the fractionating column. A higher temperature results in a higher yield of light products.
The so-called quench oil is utilized as fluid, that is the heavy oil extracted from the bottom of the fractionating column, marketable as fuel oil.
An amount of the quench oil is extracted to be used as fuel, while the remaining part is cooled by utilizing it for the process steam production and for a preheating of the crude charge.
As a consequence of cooling, the oil reaches a temperature of about 100-170.degree. C. and it is mixed at a 8:1 ratio with the fractionating column feed in order to cool the latter .
The resulting temperature at the inlet of the fractionating column is between about 170.degree. C. and 220.degree. C.
Even though an increase of this temperature is desirable for its cost-effectiveness, nevertheless it presents a number of serious unfavorable side effects, showing up particularly as a viscosity increase of the quench oil injected into the cycle.
In fact, the continually recirculating quench oil remains in the circuit at relatively high temperatures for long periods of time, and this causes its aging, because of the presence of unsaturated compounds, with polymers and rubbers formation and a resulting viscosity increase.
All the aforementioned side effects obviously entail negative repercussions on the functioning of the production plant. These are the increase of the power required by the recirculation pumps, the reduction of the thermal exchange coefficients involved in steam production, reduction that compensates and outdoes a possible contribution provided by the temperature increase, and the increase of the maintenance costs involved in the cleaning of the plant components exposed to the quench oil.
In order to avoid the aforementioned side effects, the conventional plant is operated maintaining a quench oil temperature that enables its viscosity not to overstep a value that ensures the plant operational safety. In order to avoid the occurrence of the aforementioned side effects, the conventional plant is operated maintaining a quench oil temperature so that its viscosity does not exceed a value ensuring the operational safety of the plant.
Contrary to that, an operation temperature rise in the fractionating column would make it possible to obtain a higher production of steam, as well as a higher recovery of light products, with an ensuing increase of the plant global efficiency.